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That’s the message to be gleaned from a new study that found a positive relationship in older adults between physical fitness and the size of the hippocampus—a key brain structure thought to be the central processing area for memory and learning.

Unlike other parts of the brain, neuroscientists say, it is “plastic”—malleable and dynamic. Although the hippocampus begins to degrade as you get older (its volume shrinks about 1 percent a year after age 55), it can respond to positive stimuli, suggesting that it is a “use it or lose it” organ. For example, a famous study in the 1990s found that London cab drivers, who must master many routes and names, had larger hippocampi than others.

In the current study, published in the journal Hippocampus, neuroscientist Kirk Erickson of the University of Pittsburgh wanted to see if physical exercise might have a positive effect on the hippocampus. His team tested the fitness levels of 165 adults who were over age 55 and also gave them brain scans and spatial memory tests. The findings: “The fitter subjects had hippocampuses that were about 35 to 40 percent greater in mass than sedentary individuals,” Erickson says. The result surprised him. “I wasn’t expecting that big a difference,” he says.

How much exercise do you need to get the brain benefits? Although the fitness levels of the subjects ranged from sedentary to moderately fit, “None were super athletes,” Erickson says. In other words, you don’t need to do marathons to keep up brain size, just regular exercise.

“These findings indicate that there is something older adults can do to improve their ability to remember,” says Charles Hillman, an exercise scientist at the University of Illinois at Urbana-Champaign. That something, he says, is regular physical activity, which, Erickson believes, should no longer be thought of as only good for your heart and muscles. “It directly benefits your brain and your cognitive function,” he says.

John Hanc writes about fitness and health.

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